1. Field of the Invention
The present invention relates to an optical recording medium and method, which record information by irradiating a laser beam to a recording layer so as to form a recording mark, and in particular, to a technology of forming a plurality of recording marks having different state so as to record a data at a multi-level.
2. Description of the Related Art
In a conventional optical recording medium, a method has been employed such that a data is recorded by changing a length of read signal (i.e., a length of reflection signal modulation section). On the contrary, the following method has been frequently studied, more specifically, a plurality of data is recorded to each signal having the same length by changing a depth of the read signal (i.e., a modulation degree of reflection signal) at multiple stages.
According to the above optical recording method, as compared with the case where a binary data is merely recorded by the presence of pit, a plurality of data is recorded in a depth direction, so that a signal quantity allocated to a constant length can be increased; therefore, it is possible to improve a track recording density. In general, it is well known to change a power of laser beam at multiple stages, as the method of a depth of read signal at multiple stages. Recently, a recording medium having a multiple recording layer has been proposed as the recording medium.
In this case, it is called as multi-level recording to record each data so that a modulation degree of reflection signal is variable at multiple stages.
In the above multi-level recording, a laser beam is irradiated at multiple-stage power in recording; for this reason, in particular, when the power of laser beam gradually becomes large, a problem has arisen such that a signal quality is worsened in reading.
In particular, in order to achieve a high density of the recording information of recording medium, in the case where multi-level recording is carried out in a manner of making small a recording mark, and changing a laser power at multiple stages, a signal quality has remarkably worsened in the recording mark recorded by large laser power.
In general, a diameter of the collected beam is expressed as Kxcex/NA (K: constant, xcex: laser wavelength, NA: numerical aperture of lens. For example, in a pickup used for a CD, the laser wavelength xcex is 780 nm, the numerical aperture NA is 0.50, and the diameter of collected beam is about 0.8 xcexcm. Thus, when the recording mark length is made small up to the vicinity of about 0.8 xcexcm, the above problem of signal deterioration has remarkably arisen. In fact, it is very difficult to achieve five-stage or more multi-level recording by the method of changing a laser power.
Further, likewise, the conventional concept of achieving the multi-level recording by stepwise changing the laser power is based on the following premise that the recording mark length is at least larger than a radius of focused beam (beam waist) in recording. Therefore, there is some limitation to improve a recording density unless a size of the focused beam is made small.
On the contrary, although is unknown when this application has been filed, an optical recording method has been disclosed in Japanese Patent application No. 2000-187568 filed by the same applicant as this application. More specifically, according to the above optical recording method, in place of the power of laser beam, an irradiation time to a predetermined virtual recording cell area is changed at multiple stages. By doing so, a recording mark (low optical reflectance area) having different size is formed in the virtual recording cell and giving different optical reflectance to the entire virtual recording cell; therefore, at least five-stage multi-level recording is possible.
The above optical recording method is a concept quite different from the conventional laser beam irradiation, and a recording mark may be formed by a short-time irradiation. Therefore, the case is sufficiently considered such that a recording mark smaller than a beam spot diameter is formed. In other words, multi-stage and high density multi-level recording can be realized by effectively using an area in which the recording mark has been conventionally considered as being incomplete.
The present invention has been made in order to further study the above technology. Accordingly, an object of the present invention is to provide an optical recording medium, which can achieve multi-stage (multi-level) and high read-accuracy multi-level recording by irradiating a laser beam.
The present inventors have earnestly studied optical recording medium and method; as a result, they have confirmed that it is possible to carry out multi-stage and stable high density multi-level recording.
(1) In order to achieve the above object, according to one aspect, the present invention provides an optical recording medium, including a reflective layer and a recording layer, and constructed in a manner that a recording mark is formed on the recording layer by irradiating a laser beam so as to record information,
the recording layer being continuously formed in a relative moving direction to the laser beam with plural virtual recording cells, each of which has an arbitrary unit length and a unit width perpendicular to the unit length in the relative moving direction,
plural-stage irradiation times being set with respect to the virtual recording cell so that the irradiation time becomes long successively from the first to final stages,
a laser beam power being set in at least one irradiation time of the plural-stage irradiation times so as to intermittently become larger than a predetermined threshold value when the laser beam is irradiated to the virtual recording cell, and
recording marks being formed in the virtual recording cell, the recording marks giving plural-stages different optical reflectance to the virtual recording cell.
recording marks having plural-stages different optical reflectance being formed in the virtual recording cell.
(2) Further, the present invention provides an optical recording medium, including a reflective layer and a recording layer, and constructed in a manner that a recording mark is formed on the recording layer by irradiating a laser beam so as to record information,
the recording layer being continuously formed in a relative moving direction to the laser beam with plural virtual recording cells, each of which has an arbitrary unit length and a unit width perpendicular to the unit length in the relative moving direction,
plural-stage irradiation times being set so that the irradiation time becomes long successively from the first to final stages, and the laser beam being irradiated to the virtual recording cell in accordance with the irradiation times so that recording marks being formed in the virtual recording cell, the recording marks giving plural-stages different optical reflectance to the virtual recording cell.
in at least one of the plural-stage irradiation times, the irradiated laser beam being irradiated in a state that the case where the laser beam power is large and the case where it is small are alternately continued so that the recording mark is formed.
(3) Further, the present invention provides the optical recording medium, wherein the power of laser beam is controlled in at least final stage irradiation time.
(4) Further, the present invention provides the optical recording medium, wherein in the case where an optical reflectance of non-recording state in the recording medium is set as K, the power of laser beam is controlled so that the recording mark having an optical reflectance less than 0.6K is formed.
(5) Further, the present invention provides the optical recording medium, wherein the power of laser beam is controlled from the midway time point to the termination time point in the irradiation time.
(6) Further, the present invention provides the optical recording medium, wherein the midway time point is set after the intermediate time point in the irradiation time.
(7) Further, the present invention provides the optical recording medium, wherein the irradiation time has at least five stages from the first stage to the final stage.
(8) Moreover, in order to achieve the above object, according to another aspect, the present invention provides an optical recording method, which irradiates an laser beam to an optical recording medium including a reflective layer and a recording layer, and forms a recording mark is formed on the recording layer so as to record information, comprising the following stages of:
continuously forming plural virtual recording cells, each of which has an arbitrary unit length and a unit width perpendicular to the unit length the recording layer being in a relative moving direction to the laser beam with in the relative moving direction;
setting plural-stage irradiation times with respect to the virtual recording cell so that the irradiation time becomes long successively from the first to final stages;
setting a laser beam power in at least one irradiation time of the plural-stage irradiation times so as to intermittently become larger than a predetermined threshold value when the laser beam is irradiated to the virtual recording cell; and
forming recording marks being formed in the virtual recording cell, the recording marks giving plural-stages different optical reflectance to the virtual recording cell.
(9) Further, the present invention provides an optical recording method, which irradiates an laser beam to an optical recording medium including a reflective layer and a recording layer, and forms a recording mark is formed on the recording layer so as to record information, comprising the following stages of:
continuously forming plural virtual recording cells, each of which has an arbitrary unit length and a unit width perpendicular to the unit length in a relative moving direction to the laser beam, in the relative moving direction, with respect to the virtual recording cell;
setting plural-stage irradiation times so that the irradiation time becomes long successively from the first to final stages, and irradiating the laser beam to the virtual recording cell in accordance with the irradiation times so that recording marks being formed in the virtual recording cell, the recording marks giving plural-stages different optical reflectance to the virtual recording cell.
and
reducing a change speed of optical reflectance of the virtual recording cell in the termination side of the irradiation time in at least one of the plural-stage irradiation times so that the irradiated laser beam being irradiated in a state that the case where the laser beam power is large and the case where it is small are alternately continued so that the recording mark is formed.
(10) Further, the present invention provides the optical recording method, wherein the power of laser beam is controlled so as to intermittently become larger than a predetermined threshold value in at least final stage irradiation time.
(11) Further, the present invention provides the optical recording method, wherein in the case where an optical reflectance of non-recording state in the recording medium is set as K, the power of laser beam is controlled so as to intermittently become larger than a predetermined threshold value so that the recording mark having an optical reflectance less than 0.6K is formed.
(12) Further, the present invention provides the optical recording method, wherein the power of laser beam is controlled so as to intermittently become larger than a predetermined threshold value from the midway time point to the termination time point in the irradiation time.
(13) Further, the present invention provides the optical recording method, wherein the midway time point is set after the intermediate time point in the irradiation time.
(14) Further, the present invention provides the optical recording method, wherein the irradiation time has at least five stages from the first stage to the final stage.